Terpenoid emission from Citrus sinensis (L.) OSBECK under drought stress

Periods of soil dryness are typical for the climate of Southern Europe. Sin ce drought induced restriction in carbon acquisition may reduce carbon reem ission through terpenoids, drought stress can be a factor influencing the o verall emission of terpenoids in the Mediterranean area. In this area orang e plantations are common. In order to investigate the relationship between the water status of Citrus sinensis and terpenoid emissions, monoterpene an d sesquiterpene emission rates were followed during a drought treatment and subsequent recovery. A dynamic enclosure technique was used to determine e missions of terpenoids from a branch of a young orange tree in a greenhouse . Terpenoid emissions from Citrus sinensis consisted mainly of the sesquite rpene beta-caryophyllene and the monoterpene trans-beta-ocimene. Taken toge ther these compounds accounted for 82% of the total terpenoid carbon emissi on from orange leaves. Other terpenoids were emitted in traces. Under sever e drought stress the emission of both compounds was reduced to less than 6% of the prestress level. Mild drought stress induced a decrease in the tran s-beta-ocimene emission rate whereas the beta-caryophyllene emission showed no response to slight drought stress. During the first phase of the experi ment, before severe drought stress occured, the beta-caryophyllene emission rate was closely related to temperature. The emission increased by the 5.6 fold with a 10 degrees C increase in temperature. At standard conditions ( 30 degrees C, PPFD 1069 mu mol photons m(-2) s(-1)) trans-beta-ocimene was emitted from orange leaves at a rate of 0.33 mu g g(-1) h(-1) and beta-cary ophyllene at a rate of 0.41 mu g g(-1) h(-1) given on a total dry weight ba sis, rates corresponding to 10.5 and 13.1 ng m(-2) s(-1), respectively, cal culated per total projected leaf area. With flowers the total terpenoid car bon emission was 7.8 fold the emission from the same branch without flowers . The data indicate, that severe drought stress and flowering should be con sidered in terpenoid emission inventories to avoid error in the estimation of total terpenoid emissions from vegetation in model calculations. (C) 199 9 Elsevier Science Ltd. All rights reserved.